Dryer Vent Runs: Why Bends and Sags Matter More Than Metres

Almost everything written about dryer vents is about how often to clean them. That matters, but it skips a question that determines how quickly a run fills up in the first place: what shape is the run, and where does the air slow down?

Two houses can have identical machines, identical cleaning habits and completely different outcomes, because one duct runs straight out through the wall behind the dryer and the other loops up into a roof space with three elbows and a sag in the middle.

What the duct is actually carrying

It helps to be specific about the job. A vented dryer is not simply blowing warm air outside. It is moving hot, very humid air that is carrying the fine lint the machine’s own filter did not catch — and every load of washing puts a substantial amount of water into that airstream, because that water has to leave the clothes somehow.

Two consequences follow from that. Lint stays airborne only while the air is moving quickly, so anywhere the air slows, lint drops out and stays. And because the air is close to saturated, anything that cools it makes water appear on the duct wall. Wet lint does not blow along; it sticks. That combination is the entire mechanism behind a blocked dryer vent.

Length is only half the question

Manufacturers do set a maximum duct length, and it is model-specific — the manual for your machine is the only place to get the real figure. What is more useful to understand is how that budget gets spent, because it is not spent by metres alone. Each bend is counted as adding to the total, and a tight elbow adds more than a gentle sweep.

So a run described as short can be well over the limit once its bends are counted, and a longer run with a single sweeping turn can be comfortably inside it. When a dryer takes longer than it used to and the lint filter is clean, the shape of the run is often the thing nobody has looked at.

Where a run actually blocks

Blockages are not random and they are rarely at the outside end where you can see them. They form at the points where the duct changes direction, changes shape or changes temperature.

The first is right behind the machine. The duct is usually longest there when the dryer is pulled out and crushed flat when it is pushed back, and a crushed section is both a restriction and a place lint catches. The second is any sag between fixings, which is where condensate collects and holds lint in place. The third is the elbows, where air has to change direction and slows at the outside of the turn. The fourth is the vent hood itself, where a flap stiff with old lint stops opening properly — and where, in spring, birds occasionally take an interest in a warm, sheltered opening.

What Melbourne homes tend to do to a run

Two local patterns account for a lot of bad runs. The first is the laundry in a cupboard or an internal room, common in townhouses and units, where there is no external wall behind the machine at all. The duct then has to travel — along a ceiling, through a cavity, up into the roof space — and the length and the bend count both climb.

The second is winter itself. A duct running through an unheated roof space or along an external wall is surrounded by cold air for months, so the moisture in the airstream condenses instead of leaving. That is why the state of a run at the end of a Melbourne winter is usually worse than at the end of a summer, even with the same number of loads: the same lint has been arriving, but wet rather than dry.

Venting into the roof space rather than outside deserves its own mention, because it turns a duct problem into a building problem — the moisture stays in the house instead of leaving it, and it condenses on the underside of the roof. If that is the arrangement, the run needs redirecting rather than cleaning.

Signs the run is fighting you

• Loads need a second cycle. The clearest sign, and the easiest to normalise. If the machine used to finish in one cycle and now does not, something has changed in the path rather than the machine.
• The laundry gets hot and humid. Air that cannot leave through the duct leaves around the machine instead, and it brings its moisture with it.
• Lint appearing around the dryer or at the wall. It has to be escaping from a joint, which means there is enough back-pressure to push it out.
• The outside flap does not open, or opens partly. Worth checking with the machine running — it takes ten seconds and tells you about the far end of the run.
• A hot smell. Not a maintenance signal. Stop using the machine and get the run cleared before running it again.

What can be changed, and what cannot

Where the laundry sits is usually fixed. Almost everything else about a run is not, and the practical improvements tend to be modest ones: giving the machine enough clearance that the duct is not crushed against the wall, supporting a sagging section so it no longer holds water, replacing concertina duct with something smooth-walled, easing a tight elbow into a wider turn, and making sure the hood flap moves freely.

None of that removes the need to clear the run periodically, because lint accumulates in even a well-shaped duct. What it changes is how fast it accumulates, and how much of the machine’s effort goes into pushing air through the path instead of drying the clothes. Spring, with the dryer about to get used less, is a sensible time to have the run looked at properly rather than in the middle of the wettest month of the year.

Frequently asked questions

How long is a dryer vent allowed to be?

There is no single number that applies to every machine. Manufacturers publish a maximum duct length for each model and treat every bend as adding to that total, so the honest answer is that the manual for your machine is the only reliable source. What is worth taking from it is the principle rather than the figure: the budget is spent by bends as well as by metres, which is why a run that looks short on paper can still be over the limit in practice.

Is flexible ducting a problem, or just the cheap kind?

The concertina profile is the issue rather than the price. A ridged interior gives lint a surface to catch on at every corrugation, and flexible duct also sags between fixings and gets crushed when the machine is pushed back into place. Rigid or semi-rigid ducting with smooth walls holds its shape and gives lint far less to grab, which is why it tends to be the recommendation when a run is being redone.

Why is there water in my dryer duct?

Because the air leaving a dryer is hot and full of moisture, and a Melbourne winter cools it before it gets outside. In a long or uninsulated run , particularly one through a roof space or along an external wall, that moisture condenses on the duct wall and collects at any low point. Wet lint then sticks rather than moving on, which is how a sag turns into a solid plug over a season.

Can a bad run be fixed without moving the laundry?

Often, yes. A lot of the worst runs are bad by accident rather than by design: an elbow immediately behind the machine that could be moved further along, a sagging section that needs supporting, concertina duct that could be replaced with rigid, or a vent hood with a flap that no longer opens. Those are changes to the run, not to the room. Relocating the machine is a last resort, not a first step.